Microporous films, particularly, polyolefin microporous films have been used for microfiltration membranes, battery separators, capacitor separators, fuel cell materials, and the like. They have been used particularly for separator for lithium ion batteries. In recent years, lithium ion batteries have been used for hybrid electric vehicles as well as compact electronic devices such as mobile phones and laptop personal computers.
Lithium ion batteries used for hybrid electric vehicles are required to have a higher output performance in order to produce much energy in a short period of time. In addition, lithium ion batteries used for hybrid electric vehicles are required to ensure higher safety because they usually have a large size and a high energy capacity. The term “safety” as used herein means particularly safety against short-circuit (short) of a battery due to melting of the resin used for the separator under high temperature conditions at the using time of the battery. The temperature at which short-circuit occurs inside the battery is called film rupture temperature of the separator and raising this film rupture temperature is one of the means for improving the safety of the battery.
With a view to providing a microporous film which will serve as a separator capable of coping with the situation described above, there is proposed a composite microporous film (separator for batteries) having a laminated structure obtained by stacking a polypropylene microporous film over a conventional polyethylene microporous film according to Patent Document 1.
Polypropylene is used in order to raise the short-circuit temperature. Described specifically, separators are required to maintain their film form even under high temperature condition and keep insulation between electrodes. The polypropylene resin used as a heat resistant layer, however, does not have sufficient heat resistance under severe conditions, for example, in a battery oven test that has been performed recently.
According to Patent Document 2, a synthetic resin microporous film made of polyethylene is covered with porous polymer powders made of a specific resin and therefore has improved stability at high temperature. Such a film has however still insufficient heat resistance under severe conditions, for example, in the battery oven test.
[Patent Document 1] Japanese Patent Laid-Open No. Hei 05-251069
[Patent Document 2] Japanese Patent Laid-Open No. Hei 03-291848